Add blank line after first sentence of doc comment for CheckedModeBanner (#103490)

Author: nilsreichardt

packages/flutter/lib/src/widgets/banner.dart

@@ -326,6 +326,7 @@ class Banner extends StatelessWidget {
 
 /// Displays a [Banner] saying "DEBUG" when running in debug mode.
 /// [MaterialApp] builds one of these by default.
+///
 /// Does nothing in release mode.
 class CheckedModeBanner extends StatelessWidget {
   /// Creates a const debug mode banner.